Help repairing my ZX Spectrum+

[Olipix]

I have a little experience.
I have already desoldered some chips, smaller than the Z80, with success without damaging the board.
I know it's complicated but I have to do it

I think I will have to cut the Z80 legs first, otherwise it will be very tricky.
Do you have suggestions or advices ?

Regards

[Ast A. Moore]

I think I will have to cut the Z80 legs first, otherwise it will be very tricky.
Do you have suggestions or advices ?

That seems a bit drastic given that we don’t know if the CPU is dead for sure, but I can understand your apprehension. With care, it is possible to desolder it without cutting the leads, but if you don’t feel confident enough, it’s best to sacrifice the CPU rather than the PCB.

A hand-held solder sucker and desoldering braid can go a long way. Apply a little bit of fresh solder to the joint and then, while still heating it with the iron, quickly press the nozzle of the solder sucker to the joint, remove the iron tip, and release the spring-loaded piston at the same time. This all must be done in one swift and decisive motion. The nozzle of the solder sucker must be pressed to the joint firmly enough to ensure a good seal.

[Olipix]

Hello guys

I've changed the Z80 (I even managed to preserve the original one) and I've changed the ULA and...
the problem is still the same : multi colored artefacts at boot + border that change color when Reset is pressed....

Any recommandations ?
Should I change the ROM too ?

Thanks for any advice

[Ast A. Moore]

Yes, replacing the ROM is a logical next step. If that doesn’t help, I’d really start looking into shorts/broken traces.

[1024MAK]

Before desoldering the ROM, it’s possible to disable it (if it’s not completely dead).

On the edge connector, pin 25 on the underside can be connected to +5V to tell the ROM to ignore any reads. Alternatively you can connect to the appropriate lead of resistor R33 (680Ω). Make sure you connect to the lead that goes to edge connector pin 25.

In this state the ROM chip should turn off its output pins. Now the Z80 will read whatever value the data bus floats to. Which, if there are no misbehaving chips fitted, should be 255 (FFh or 0xFF) due to ‘pull-up’ resistors R9 to R16.

As far as the Z80 is concerned 0xFF means RST 38h. When the Z80 executes this instruction, the current PC value plus one is pushed onto the stack, then the PC is loaded with 38h (0x38). The Z80 then fetches the instruction at this address which is again 0xFF.

The end result is that all RAM will fill up with the values that are being pushed to the stack, as the stack pointer works its way through all possible addresses before wrapping around and then doing it all again.

If the screen RAM and the ULA are working, this will produce a distinctive diagonal pattern on the screen.

Mark

[Olipix]

Excellent.

So, to test this I only have to solder a wire between pin 25 and +5v ?

[1024MAK]

Excellent.

So, to test this I only have to solder a wire between pin 25 and +5v ?

No, don’t solder to the edge-connector

If you have a spare female edge-connector socket, or a simple interface like a joystick interface, use either a test clip or solder to the relevant pins on the edge-connector socket.

If you don’t have anything suitable, use the resistor. First, with the power off and disconnected, switch your multimeter to the 200 ohm resistance range (or equivalent). Now test between each lead of resistor R33 and pin 25 on the underside (of the board) of the edge-connector contacts. When your meter shows a resistance of less than 1 ohm, you now know you have the correct lead of resistor R33. You can now either attach a test clip and then connect to any +5V point. Or you can solder a wire to the same resistor lead or the solder connection on the underside of the board.

Please do double check your work before powering on. Making a mistake and applying +5V to the wrong place would not be a good thing to do...

Mark

[Olipix]

Ok, I've linked the right leg of the R33 and +5v, and this is what it gives :

https://youtu.be/lPLKieSuBcc

Any thoughts ?

[1024MAK]

Yeah, there is no sign of the Z80 writing to screen memory

I understand you have a multimeter. But don’t have or have access to an oscilloscope.

Do you have a logic probe or a logic analyser?

Mark

[Olipix]

Thanks Mark for the reply

Indeed I have nos oscilloscope right now
But I have this, if it can help...

[1024MAK]

I can’t see the picture that you tried to post. Can you use the “Add image to post” function (just below the text edit box) to include pictures please.

Mark

[Olipix]

Sorry. I did as you said, but the URL was in fact private.

here is the pciture of the tool I have :

[Olipix]

Hi guys !

Just to let you know that I managed to get my ZX Spectrum+ working.
It was the Z80 !

Thanks for all your help

Now I'm onto trying to save another Spectrum+...

[Ast A. Moore]

Glad we could help!